You got me to do some research Mos.  But try as I could, I couldn't find a good pic or any information on how the bubble is used on those Davis inclinometers.  Just curious if it's a typical two marks and you center the bubble between them or if it's graduated like machinists levels?
 

After some Ebay shopping, I think they're out of my price range but I was still curious about the precision of them.